This invention relates to flexible, pressure activated switches and, in particular, to such a switch which is especially suitable for use in water or other fluid systems where pressure of the fluid activates the switch.
A pressure actuated switch is disclosed in commonly assigned U.S. patent application Ser. No. 176,538, filed Apr. 1, 1988 in the name of Henry L. West, and entitled "Fluid Pressure Switch Having Venting Means for Dispensing Back Pressure". The switch disclosed in such application comprises a plurality of layers bonded together to form a unitary switch. Two layers have electrically conductive elements thereon that are separated by another layer having openings aligned with the conductive elements so that pressure exerted against the switch surface deflects the layers to bring the electrically conductive elements into contact, thereby closing the switch. In order to avoid undesirable pressure increases in the openings when the switch is closed, the openings are vented through the top of the switch into large, closed chambers in a back-up plate so that closing the switch results in only nominal increase in the fluid pressure. In order to avoid leakage of water or other contaminants into the switch, the vent chamber in the back-up plate is surrounded by an O-ring which bears against the switch to seal the connection therebetween. The electrical conductors in the switch are either dome-shaped elements or comprise a strip of smooth stainless steel.
While the switch disclosed in the aforementioned application has generally worked satisfactorily, it appears that improvements can enhance the long term performance of the switch. For example, when used with a water or other liquid system, the liquid sometimes enters the vent openings even though they are firmly sealed by O-rings in the back-up plate. When this occurs, the switch is shorted out and fails. Also, the O-ring used between the switch and back-up plate, together with an O-ring on the opposite side of the switch, may deform the switch over a period of time, thus degrading its performance. Also, in one embodiment disclosed in the prior switch, the circular domes used for the electrical contacts contain sharp edges that can cut into the adjacent plastic material of the switch. This problem is avoided in another embodiment of the switch which uses a smooth strip of steel or other conductive material in place of the domes. However, even the steel strips can deteriorate over a period of time under extended exposure to water at pressures of, for example, 100 psi, until the trip pressure changes significantly. This problem is believed to be caused by creep of the adhesive between the steel layer and the adjacent plastic layers. Also, failure of the adhesive bonding the various layers together can result in leakage of water or other fluids into the switch to cause shorting or other defects in the switch.